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LM3553: With low VLED (~1.6V, single LED)

David D
Intellectual 580 points
Part Number: LM3553

Hi,

I have an existing design using the LM3553 to drive a single LED.

In the next modification of the disign, the LED is changed to one with a lower Vf.

I'd like to know whether I still can remain with the same LED driver for the modified design?

(Basically, the question is whether the LM3553 can work as a step down converter?) 

Design parameters:

Vin: 2*AA batteries (i.e. ~3V)

Output: Single LED: Vf=~1.6V, Iout=~0.3A.

Thanks,

David.

  • 0
    TI__Intellectual 1940 points
    David,
    The LM3553 will not work in a step down mode as it only can operate in a pass (no switching) or boost mode. If you are ok with the linear drive performance (where the efficiency is not as good), you might be able to make the LM3553 work, if you omit the inductor and schottky diode and tie off the OVP pin. That said, are you ok using chipscale packages? If so we have some newer devices that are much smaller than this device and have some improved features but they too would only operate linearly as they do not have a dedicated buck converter paired with a current source.

    Greg
  • 0 in reply to Greg Lubarsky
    Intellectual 580 points
    Hi Greg,

    Thanks.

    I'd like to ask again to make sure there is indeed an issue here.

    I have a current design with a LED that has Vf=~2.7V.

    I'd like to replace the LED to one having Vf=~1.6V.
    Is this a problem?
    Does it even means that the LM3553 will be required to work in step-down mode? Or maybe the voltage on D1,D2 will raise accordingly so the Vout will still be higher than Vin and the mode will be step-up?
    Is there any limitation on the Vf of the LED?


    Thanks,
    David.
  • 0 in reply to David D
    TI__Intellectual 1940 points

    David,

    One concern would be on-chip thermal dissipation but in your case it is probably not an issue due to the low VIN and relatively low led current.  Let me be clear in that this device does not ever work in step-down mode.  When VIN is greater than VLED+VD1,D2, the boost will stop switching.  In this situation, the voltage on D1 and D2 will be come unregulated and will increase to VIN -Vschottky-VLED-(ILEDxRINDUCTOR).  This is ok because the current sink will maintain current regulation.  As the VIN min for this device is 2.7V and your LED is 1.6V, the part should in fact never boost.  This means that in theory you should be able to remove the inductor and schottky diode and just drive the anode of the LED from the battery supply only.  I am not sure there is really a limitation on the LED voltage other than at higher currents, if VIN - VLED is high, the thermal dissipation becomes high and the part could run above the maximum junction temperature at elevated ambient temps.

    If you already have an existing design, have you already tried switching the LEDs to see how things perform?

    Greg